The nature of stem cells

Within the mammalian body there is a ranking system of cells from the undifferentiated to the highly specialised cell types and tissues, e.g. liver, brain, lung, blood, skin, etc., which have arisen by the process of cellular differentiation. Cellular differentiation occurs by a variety of biological processes that involve the switching on and off of specific genes, of cell signalling and, especially, where the cell is situated in the body. When a cell achieves terminal differentiation it cannot reproduce itself. Yet, it has long been recognised that systems and tissues of the adult body must have some ability to replace cells such as cells of the blood system (haematopoietic cells) and skin cells (epidermal tissue). The body does, however, retain certain undifferentiated cells within tissues and such cells are termed tissue or adult stem cells (TS).

What are stem cells? Stem cells are undifferentiated cells that have the capacity to self-renew and to achieve multilineage differentiation. Within a particular cell population they can remain in an undifferentiated state and, as such, do not have any specialised function. However, when required they can be induced to differentiate into specific cell types. Stem cells have specific enzymes and cell-specific antigens and have the ability to express developmentally regulated genes. Stem cells have been characterised as totipotent, pluripotent and multipotent.